The present invention relates to a demodulator arrangement, a method for demodulation of data symbols performed by such a demodulator arrangement, and a telecommunication system comprising such a demodulator.
Such a demodulator arrangement and method to demodulate are already known in the art, e.g. from the article xe2x80x98Demodulator With FEC Decoder Has On-Chip Timing Recoveryxe2x80x99 from the author Paul McGoldrick. This article was published on pages 55-64 of the magazine xe2x80x98Electronic Designxe2x80x99 of Sep. 3, 1996. In this article, a family of demodulators is described suitable for reception and demodulation of television signals transferred over a cable, for instance an optical fibre or coax cable. Data symbols are modulated in the known system in accordance with a constellation diagram that is shown in FIG. 2 on page 58, and that is described via the tables on page 56 of the cited article. The drawn constellation diagram contains 64 constellation points and has a rectangular shape. The same constellation diagram evidently is used in the demodulator of the known system to demodulate received data symbols. The known demodulator, shown in FIG. 1 of the above mentioned article, contains a fully coherent detector which determines the amplitude and phase of the data symbols independently of one another. To be able to accurately determine the amplitude of the received data symbols, the demodulator includes an automatic gain control loop which compensates for changes in the amplitude attenuation of the cable whereover the information is transferred. In addition, to be able to accurately determine the phase of received data symbols, the known demodulator includes a timing recovery loop which is described in detail in the paragraph xe2x80x98Jittering Towards Zeroxe2x80x99 on the lower half of page 58 of the cited article. The timing recovery loop has the task to establish a phase reference for the incoming information bursts. As will be appreciated by any skilled person and as can be understood from the paragraphs xe2x80x98System Overviewxe2x80x99 on pages 55-56 and xe2x80x98Jittering Towards Zeroxe2x80x99 on page 58 of the cited article, the automatic gain control loop and the timing recovery loop have a significant share in the hardware complexity of the known television signal demodulator. Moreover, such a coherent detector typically requires a fairly long acquisition time when working in blind acquisition mode. Using a training sequence at the start of everey information burst may reduce the acquisition time of coherent detectors to a few symbols but the current commercially available QAM-receivers are not suited for this application.
It is an object of the present invention to provide a demodulator arrangement suitable to receive and demodulate data symbols transmitted as part of information bursts over a transmission medium with stable attenuation characteristics, but whose hardware complexity is significantly reduced in comparison with that of the known demodulator arrangement and whose acquisition time is reduced significantly. In addition, it is an object of the present invention to provide a related demodulation method and telecommunication system.
According to the invention, this object is realised by the demodulator arrangement defined in claim 1, the method to demodulate data symbols defined in claim 7, and the telecommunication system defined in claim 8.
Indeed, in case bursty information is transferred over a transmission channel with in time slowly varying attenuation characteristics, coherent amplitude detection will be successful without the need for an automatic gain control loop. If the information is transferred over a channel, for instance a copper twisted pair cable, a coax cable or an optical fibre, whose gain remains stable in time, no automatic gain control is required to enable coherent amplitude detection. It is sufficient that the attenuation of the channel is measured once at initialisation of the system. On the other hand, by using a differential phase detector which detects the phase of a data symbol by measuring the phase difference with the phase of the previously received data symbol, no timing and/or carrier recovery loops are required in the demodulator according to the present invention, although the traffic has a bursty character. Differential phase detection is a known technique used in fully differential demodulator arrangements which are used to detect data symbols sent over transmission channels with fast varying amplitude and phase characteristics such as radio links. Such a fully differential detector for instance is known from pages 324-327 of the book xe2x80x98Modern Quadrature Amplitude Modulation, Principles and Applications for Fixed and Wireless Communicationsxe2x80x99, written by William Webb and Lajos Hanzo, and published by Pentech Press, London. Summarising, according to the present invention differential phase detection is combined with coherent amplitude detection in a detector used to receive bursty information transferred over a gain stable link. Thereby, control loops for amplitude and phase are avoided so that the demodulator""s hardware complexity is reduced significantly.
It is to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noticed that the term xe2x80x98coupledxe2x80x99, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x98a device A coupled to a device Bxe2x80x99 should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
In a particular embodiment of the present invention, a large constellation scheme is used as defined in claim 2.
An additional feature of the present invention is defined in claim 3.
In this way, by using a circular constellation diagram wherein phases are distributed equidistantly and wherein amplitude levels are equidistant, robustness for noise of the coherent amplitude, differential phase detector according to the present invention is minimised.
A further feature of the demodulator arrangement according to the present invention is defined in claim 4.
In this way, by using a so called staggered constellation diagram instead of a linear constellation diagram, the distance between constellation points is increased which even more improves robustness for noise.
Furthermore, an embodiment of the present invention is adapted for upstream transmission in a time multiplexed way over a hybrid fibre coax network as defined in claim 5.
Indeed, the hybrid optical fibre/coaxial cable transmission medium of a hybrid fibre coax network has a stable gain in time. Moreover, because the medium is shared in a time division multiplexed way, upstream transmission occurs in bursts which makes applicability of the present invention to hybrid fibre coax systems very suitable.
Alternatively, an embodiment of the present invention is adapted for downstream transmission in an hybrid fibre coax system, as defined by claim 6.